This application claims the benefit of Korean Patent Application No. 2001-23749 filed on May 2, 2001, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method of discriminating whether a light spot is formed on a land or a groove of a land-groove type optical disc, and to an optical recording and/or reproducing apparatus employing the same.
2. Description of the Related Art
For reliable tracking and focusing with respect to a land-groove type optical disc, there is a need to discriminate whether a current light beam spot is positioned on a land or in a groove. This is because compensation of an offset for a tracking error signal caused by objective lens shifting in tracking by a push-pull method and compensation of a focus offset should each be applied differently to lands and grooves.
Referring to FIG. 1, a conventional optical pickup to discriminate lands and grooves includes a light source 1, a collimating lens 3 collimating a diverging light beam emitted from the light source 1, a hologram element 5 splitting a light beam by diffraction, a polarizing beam splitter 7 transmitting or reflecting an incident light beam according to the polarization of the incident light beam, a quarter-wave plate 9 changing the polarization of an incident light beam, an objective lens 11 focusing an incident light beam to form a light spot on an optical disc 10, and a photodetector 15 receiving a light beam reflected/diffracted by the optical disc 10.
The hologram element 5 diffracts an incident light beam into a main light beam and first and second sub-light beams. As shown in FIG. 2, the hologram element 5 has first through fourth sections 5a, 5b, 5c, and 5d and is designed such that a light beam passing through the first and third sections 5a and 5c has a phase difference of 90xc2x0 with respect to a light beam passing through the second and fourth sections 5b and 5d. Thus, the phase of a portion of the first sub-light beam, i.e., a +1st order light beam, having passed through the first and third sections 5a and 5c leads that of a portion of the first sub-light beam having passed through the second and fourth sections 5b and 5d by 90xc2x0. In contrast, the phase of a portion of the second sub-light beam, i.e., a xe2x88x921st order light beam, having passed through the first and third sections 5a and 5c lags that of a portion of the second sub-light beam having passed through the second and fourth sections 5b and 5d by 90xc2x0.
Referring to FIG. 3, the objective lens 11 forms a main light spot 1a and first and second sub-light spots 1b and 1c on the optical disc 10 by focusing the main light beam and the first and second sub-light beams split by the hologram element 5.
Referring to FIG. 4, the photodetector 15 includes a main photodetector 15a and first and second sub-photodetectors 15b and 15c receiving the main light beam and the first and second sub-light beams reflected by the optical disc 10, respectively. The main photodetector 15a includes four divided plates A, B, C, and D, the first sub-photodetector 15b includes four divided pates E, F, G, and H, and the second sub-photodetector 15c includes four divided plates I, J, K, and L.
Supposing that signals output from the four divided plates A, B, C, and D of the main photodetector 15a are designated by a, b, c, and d, respectively, signals output from the four divided plates E, F, G, and H of the first sub-photodetector 15b are designated by e, f, g, and h, respectively, signals output from the four divided plates I, J, K, and L of the second sub-photodetector 15c are designated by i, j, k, and l, respectively, and a reproduction signal RFS, a land/groove (L/G) signal, and a tracking error signal TES are expressed as formulae (1):
RFS=a+b+c+d 
L/G signal=(e+g+j+)xe2x88x92(f+h+l+k) 
TES=(a+b)xe2x88x92(c+d)xe2x80x83xe2x80x83(1) 
The tracking error signal TES and the land/groove (L/G) signal detected as described above have the same period but a phase difference of 90xc2x0, as shown in FIG. 5. When the tracking error signal TES has a value of zero, the land/groove (L/G) signal has a maximum or minimum value, respectively. Thus, whether a light spot is focused on a land or groove of the optical disc can be determined. However, the conventional optical pickup described above uses three light beams split from a light beam emitted from the light source 1 so that there is a problem with light utilization efficiency.
Accordingly, it is an object of the present invention to provide a method of discriminating whether a light spot is focused on a land or a groove of an optical disc using an optical element that can generate an effect of thickness variation of the optical disc, which can achieve high light utilization efficiency by using a single light beam, and an optical recording and/or reproducing apparatus employing the same.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing and other objects of the present invention are achieved by providing a land/groove discriminating method comprising: emitting a light beam onto an optical disc having lands and grooves through an optical element causing and varying an effect of thickness variation of the optical disc; detecting a light beam reflected/diffracted from the optical disc by dividing the reflected/diffracted light beam into inner and outer light beam portions; and discriminating between a land and a groove of the optical disc by detecting variation in a difference signal between detection signals from the inner and outer light beam portions while varying the degree of the effect of thickness variation of the optical disc by driving the optical element.
The above and other objects of the present invention may also be achieved by providing an optical recording and/or reproducing apparatus comprising: an optical pickup comprising an optical element on a traveling path of a light beam emitted from a light source to be focused on an optical disc by an objective lens, the optical element causing and varying an effect of thickness variation of the optical disc, and a photodetector device which detects a light beam reflected/diffracted by the optical disc by dividing the reflected/diffracted light beam into inner and outer light beam portions; and a signal processing unit which detects a difference signal by subtracting a detection signal from at least one outer light beam portion from a detection signal from the inner light beam portion and discriminates between a land and a groove of a land/groove type optical disc by detecting variation in the difference signal while varying the degree of the effect of thickness variation of the optical disc by driving the optical element.
The above and other objects may also be achieved by providing that the optical element compensates for spherical aberration caused by thickness variation of the optical disc. Additionally, it may be provided that discrimination between a land and a groove of the optical disc is determined by an increase or decrease in the DC level of the difference signal.
Further, it may be provided in an embodiment of the present invention that the photodetector device detects the light beam by dividing it into inner and outer light beam portions in a direction corresponding to a radial direction of the optical disc. Further, it may be provided that the photodetector device is formed of many different types of photodetectors that will provide the desired results of the present invention, such as, for example, an 8-sectional photodetector.
Alternatively, in an embodiment of the present invention, the photodetector device may be constructed to detect the light beam by dividing it into a circular or rectangular inner light beam portion and an outer light beam portion surrounding the circular or rectangular inner light beam portion. As another example, the photodetector device maybe a 12-sectional photodetector including four inner divided plates receiving an inner light beam portion and eight outer divided plates surrounding the inner divided plates receiving an outer light beam portion.